The present invention is intended to improve both the economic and operational aspects of electrolytic units such as chlor-alkali cells which employ membrane separators between anolyte and catholyte chambers; and more specifically, to the periodic rejevenation of such membranes which have been contaminated by multivalent metallic impurities found in the normal brine feed to such a chlor-alkali cell.
The electrochemical industry has long been concerned with the detrimental effects caused by impurities found in feed streams to the electrolytic cells. Initially, such chlor-alkali cells of the membrane type would be merely operated using the impure feed brine and operation of the cell would continue until the current efficiency dropped to an unacceptable level. Thereupon, the cell operation would be stopped and the membrane would either be replaced by a new membrane or the membrane which had been clogged with the impurities would be acid washed to remove such impurities on and/or in the membrane to rejuvenate the membrane. Such acid washing was effective provided the membrane was not irreparably damaged aged by contamination, but obviously caused down time for the electrolytic cell being cleaned.
Thereafter, another means of extending cell operation was developed which sequestered the polyvalent metallic cations in the brine prior to coming in contact with the membrane in a chlor-alkali cell. This development as fully described in U.S. Pat. No. 3,793,163 consisted of adding phosphoric acid or the like to the brine feed so as to sequester the polyvalent impurities which decreased the life expectancy of the membrane used in the cell. Such treatment effected excellent results when the impurities found in the brine feed consisted of hardness impurities (calcium and magnesium), and the like but was wholly ineffective with respect to such impurities as mercury. Likewise, other polyvalent impurities found in the brine such as iron, manganese, cobalt, nickel, copper, zinc, aluminum and the like would not be kept in their entirety from entering the membrane. Thus, although such phosphoric acid treatments lengthen the normal life of a membrane, resort still had to be made to discontinuing operation of the cell and acid washing the membrane or replacing the same with a new membrane. Thus, continuous operation still was not possible and nonproductive periods were a necessity so as to clean contaminated membranes.
The other obvious answer to this dilemma was to purify the brine of its contaminating polyvalent metallic impurities prior to feeding it to the electrolytic cell. Numerous method, of course, were known to so purify the brines. For example, East German Patent No. 49,347 published Aug. 5, 1966 discloses the purification of alkali metal salt solutions of their contaminating trace metal impurities by the use of ion exchange which quantitatively removes such trace metal impurities so that the most sensitive detection methods could not detect them. Particularly disclosed in the East German Patent is the use of iminodiacetic acid type ion exchange resin which would be similar to Dowex A-1. A more detailed discussion of the use of this type ion exchange resin to remove trace metal impurities can be found in Japanese Patent Application Public Disclosure No. 86,100 published July 28, 1976.
Purification of the brine feed prior to electrolysis does in fact maximize the life expectancy of a membrane in a chlor-alkali cell. However, the cost of such purification is extremely high and it has in fact prevented such treatment from being practiced commercially since the cost per unit of product produced remains cheaper in the practice of the phosphoric acid brine treatment in spite of the required shut downs to clean and/or change membranes.